Internal combustion engines, which operate with a lean mixture, i.e., at a lambda value greater than 1, in phases, are generally provided with an external exhaust-gas recirculation system. Using an exhaust-gas recirculation duct that leads into the intake manifold of the internal combustion engine, burned air/fuel mixture from an exhaust branch of the internal combustion engine is mixed with fresh air supplied to the engine. The amount of recirculated exhaust gas is controlled by an exhaust-gas recirculation valve in the exhaust-gas recirculation duct.
The external exhaust-gas recirculation system may be affected by various disturbances. For example, the exhaust-gas recirculation duct may be narrowed by deposits of soot, oil residues, or condensate. Since an incorrect amount or an excess of recirculated exhaust gas may negatively affect the NOx emissions of the internal combustion engine and changes the torque generated by the internal combustion engine, an adaptation of the exhaust-gas recirculation is frequently carried out by the electronic engine control unit, with the goal of compensating for the soiling of the exhaust-gas recirculation duct by correcting the degree of opening of the exhaust-gas recirculation valve. In addition, legislation requires an on-board diagnosis of the exhaust-gas recirculation system, the diagnosis generating an error message in the case of disturbances having a negative effect on emissions.
German Patent Applications DE 100 41 073 A1 and DE 101 15 750 A1 describe a method for adapting and diagnosing the exhaust-gas recirculation system, where the intake-manifold pressure is measured, on one hand, by a pressure sensor. On the other hand, the intake-manifold pressure is modeled in view of the degree of opening of the exhaust gas recirculation valve and additional parameters, such as the engine speed, the degree of opening of the throttle valve, the camshaft position, and the effect of a fuel-tank vent line. A malfunction of the exhaust-gas recirculation system may be deduced from the difference between the measured and the modeled value of the intake-manifold pressure.
Conventionally, internal combustion engines, which are operated with a lean mixture, are equipped with adsorption catalysts, in particular NOx-adsorption catalysts. This is particularly the case with spark-ignition engines, which have direct gasoline injection and are driven using stratified operation.
Adsorption catalysts have the characteristic that the stored pollutant, e.g., NOx, must be discharged with the aid of periodic, brief, rich operation, i.e., operation of the internal combustion engine at a lambda value less than 1. This is referred to as regeneration. The engine control unit triggers instances of regeneration under various circumstances, in particular when it is detected that the NOx trap of the adsorption catalyst is full. If an NOx sensor is installed downstream from the adsorption catalyst, then, e.g., the engine control unit detects a full NOx trap, when the NOx sensor measures a high NOx concentration greater than a first threshold value or a high NOx mass flow rate greater than a second threshold value in the exhaust gas leaving the adsorption catalyst. It is also convention for the engine control unit to model the NOx level of the NOx trap of the adsorption catalyst on the basis of a modeled NOx concentration in the untreated exhaust gas upstream from the adsorption catalyst. The end of regeneration is a function of the signal of the sensor, which is downstream from the adsorption catalyst and may be, as described, an NOx sensor or an oxygen sensor having a two-point characteristic. The regeneration is ended, when the sensor detects that no more NOx is discharged from the NOx trap of the adsorption catalyst, but rather that the rich exhaust gas supplied to the adsorption catalyst for regenerating the NOx trap of the adsorption catalyst flows through the adsorption catalyst substantially unaltered.
In this case, e.g., an oxygen sensor, which has a two-point characteristic and is positioned downstream from the catalyst, would rapidly switch from detecting a lean exhaust gas to detecting the exhaust gas, which is now rich and no longer needed for regeneration. According to today's state of the art, even NOx sensors output, in addition to the NOx-concentration signal, a signal which has a two-point characteristic and may be used for ending the regenerating phase.
A method and a model for modeling a discharge phase of a nitrogen-oxide adsorption catalyst are described in PCT Application WO 02/14659. In the case of internal combustion engines, which can be operated with a lean fuel-air mixture (lambda greater than 1), NOx adsorption catalysts are used in order to store the emitted NOx expelled by the internal combustion engine during lean operation. In this case, the NOx adsorption catalyst is in the so-called storage phase. The efficiency of the NOx adsorption catalyst decreases with increasing duration of the storage phase, which leads to an increase in the NOx emissions in back of the NOx adsorption catalyst. The reason for the reduction in the efficiency is the increase in the nitrogen oxide (NOx) level of the NOx adsorption catalyst. The NOx level can be monitored, and after a specifiable threshold value is exceeded, a discharge phase or regenerating phase of the NOx adsorption catalyst can be initiated. A nitrogen oxide (NOx) storage model can be used for determining the NOx level of the NOx adsorption catalyst. NOx storage models are generally conventional. In an NOx storage model, the NOx level can be modeled from the parameters describing the operating point of the internal combustion engine (e.g., the supplied mass of fuel or mass of air, the torque, etc.).
During the discharge phase, a reducing agent, which reduces stored nitrogen oxides to nitrogen (N2) and carbon dioxide (CO2), is added to the exhaust gas of the internal combustion engine. For example, hydrocarbons (HC), carbon monoxide (CO), and/or hydrogen (H2), which can be produced in the exhaust gas using a rich setting of the fuel-air mixture (homogeneous operation of the internal combustion engine), can be used as reducing agents. HC, CO, and H2 are also referred to as rich gases. As an alternative, urea can also be added to the exhaust gas as a reducing agent. In this context, ammonia from the urea is used to reduce the nitrogen oxide to nitrogen and carbon dioxide. The ammonia may be recovered from a urea solution by hydrolysis.
Towards the end of the discharge phase, a large part of the stored nitrogen oxide is reduced and less and less of the reducing agent encounters nitrogen oxide, which it can reduce to nitrogen and carbon dioxide. As a result, the concentration of reducing agent in the exhaust gas in back of the NOx adsorption catalyst increases towards the end of the discharge phase, and the concentration of oxygen in the exhaust gas in back of the NOx adsorption catalyst decreases. By analyzing the exhaust gas in back of the NOx adsorption catalyst with the aid of suitable exhaust-gas sensors (e.g., O2 sensor or NOx sensor), the end of the discharge phase may then be initiated when the majority of the nitrogen oxide has been discharged from the NOx adsorption catalyst. In addition, the NOx level of the NOx adsorption catalyst can be determined with the aid of a discharge model, and that the end of the discharge phase can therefore be determined with the support of a model.
The end of the regenerating phase should be determined as accurately as possible, since a regenerating phase that is too short does not completely empty the NOx adsorption catalyst and the NOx emissions consequently increase. On the other hand, a regenerating phase that is too long results in an increase of reducing-agent emissions (rich gases or urea). Both an increase in the NOx emissions and an increase in the reducing-agent emissions are environmentally harmful and should therefore be reduced to a minimum.
The use of suitable exhaust-gas sensors for analyzing the exhaust gas in back of the NOx adsorption catalyst and determining the end of a regenerating phase is relatively complicated and expensive. In the case of the model-based method for determining the end of the regenerating phase, a mass flow rate of reducing agent is determined from the composition (lambda) of the fuel-air mixture and the mass flow rate of air supplied to the internal combustion engine for combustion. Using a temperature-dependent factor, this is converted to a mass flow rate, as a function of which a reduction in the NOx stored in the NOx adsorption catalyst during lean operation of the internal combustion engine is calculated.
This modeling has the disadvantage that it is relatively inaccurate and only conditionally useful for determining the end of a regenerating phase. The reason for this is, in particular, that during the regenerating phase, the reducing agent, in addition to reducing the stored NOx, reduces O2 that is also stored. The stored gas, NOx or O2, which is then actually reduced at a particular time during the regenerating phase, is a function of the construction type of the NOx adsorption catalyst. Therefore, the discharge model known from the related art does not allow one to deduce which gas is reduced at which time, and by how much, during the regenerating phase.
In this context, PCT Application WO 02/14659 describes that the O2 trap is modeled by a first integrator for oxygen (O2) and the NOx trap is modeled by a second integrator for nitrogen oxides (NOx), and that the first integrator and the second integrator are proportionally acted upon by the reducing-agent mass flow rate in accordance with a distribution factor, the distribution factor being ascertained as a function of the content of the O2 trap and the content of the NOx trap of the NOx adsorption catalyst. In so doing, the mass flow rate of reducing agent is ascertained from the composition of the fuel-air mixture and a mass flow rate of air supplied to the internal combustion engine for combustion. The temperature in the NOx adsorption catalyst may be taken into account during the determination of the O2 storage capacity of the O2 trap. The following processes take place during the regenerating phase of the adsorption catalyst: the reducing agent reduces the stored nitrogen oxides to nitrogen and carbon dioxide. These substances exit the catalyst, which means that an excess of oxygen occurs in back of the catalyst during the regenerating phase, although the internal combustion engine is operated with a rich fuel-air mixture and, therefore, oxygen deficiency.
German Patent Application No. DE 198 43 879 A1 describes a method for operating an internal combustion engine, in whose exhaust region an NOx adsorption catalyst is situated. In a first operating phase in which the internal combustion engine is driven with a lean mixture in the scope of a stratified cylinder charge, the produced NOx is stored in the NOx adsorption catalyst. In a second operating phase in which the internal combustion engine is operated with a stoichiometric or rich mixture within the scope of a homogeneous cylinder charge, the NOx adsorption catalyst is regenerated. An NOx sensor situated in back of the NOx adsorption catalyst detects an increasing NOx concentration in the exhaust gas during the storage phase. A shift into the regenerating phase is initiated, as soon as the NOx concentration exceeds a predefined threshold value. In other exemplary embodiments, a shift occurs from the storage phase to the regenerating phase, when the mass flow rate of NOx or the integral of the mass flow rate of NOx in back of the NOx adsorption catalyst exceeds a predefined threshold value in the storage phase. The mass flow rate of NOx in back of the NOx adsorption catalyst may be calculated from the NOx-Sensor signal, the mass flow rate of exhaust gas, which may be determined, for example, from the measured mass flow rate of intake air, and a constant factor that represents the molar mass.
German Patent Application No. DE 197 39 848 A1 also describes a method for operating an internal combustion engine, in whose exhaust region an NOx adsorption catalyst is situated. A shift from the storage phase to the regeneration phase is undertaken as a function of the mass of NOx stored in the NOx adsorption catalyst. The mass is determined from the integral of the NOx mass flow rate, which is obtained from the measured air mass flow rate or from the known load of the internal combustion engine. If desired, the speed of the internal combustion engine and/or the exhaust-gas lambda and/or the catalyst temperature and/or the saturation behavior of the catalyst may be taken into account, as well.
German Patent Application No. DE 100 36 453 A1 also describes a method for operating an internal combustion engine, in whose exhaust region an NOx adsorption catalyst is situated. The shift from the storage phase to the regenerating phase occurs as a function of the mass of NOx stored in the NOx adsorption catalyst. The mass flow rate of NOx occurring in back of the NOx adsorption catalyst is both calculated on the basis of a model of the NOx adsorption catalyst and ascertained from the signal of an NOx sensor. The model of the NOx adsorption catalyst is corrected by comparing the two mass flow rates.
German Patent Application No. DE 103 13 216.3 describes a method for operating a nitrogen-oxide adsorption catalyst situated in the exhaust region of an internal combustion engine, where the correct time at which a shift should be made from a storage phase into a regeneration phase is ascertained. In this context, the mass of NOx actually stored in the NOx adsorption catalyst or the mass flow rate of NOx occurring in back of the NOx adsorption catalyst may be ascertained in the storage phase with comparably high accuracy, with the aid of an integrator.
An adsorption catalyst is a component, which is relevant to the exhaust gas and must be diagnosed. To this end, the NOx storage capacity of the adsorption catalyst is determined. As described, two measuring methods are generally known for this:
According to a first method, a full NOx reservoir can be detected, as described, e.g., with the aid of an NOx sensor downstream from the adsorption catalyst. The level of NOx present in the adsorption catalyst at this instant is modeled, for example, as described in PCT Application W002/14659 A1. Consequently, the level of NOx stored in the adsorption catalyst during the storage phase, i.e. during lean operation of the internal combustion engine is ascertained by the first method.
PCT Application W002/14659 A1 also describes that the end of the regenerating phase can be ascertained with the aid of the mass flow rate of the reducing-agent necessary for the regeneration of the adsorption catalyst, the oxygen storage capacity of the adsorption catalyst being taken into consideration with the aid of a distribution factor.
In addition, internal combustion engines having a two-branch exhaust system are conventional, where, in each instance, an exhaust-gas recirculation duct having its own exhaust-gas recirculation valve runs out of both exhaust-gas banks and returns to the intake manifold. In such a system, the problem of separately adapting or diagnosing the soiling of the two exhaust-gas recirculation ducts or the two exhaust-gas recirculation valves has not been solved to date. Using the above-described, conventional methods for diagnosing and/or adapting the exhaust-gas recirculation system, to date, it has only been possible to adapt or diagnose the entire exhaust-gas recirculation system. However, an incorrect amount or an excess of recirculated exhaust gas cannot be specifically attributed to the malfunction of one of the two exhaust-gas recirculation valves.